Inkjet printing is prominent because of its non-impact, low-noise characteristic, its use of plain paper, and its avoidance of toner transfers and fixing. Inkjet printing mechanisms can be categorized as either continuous or drop-on-demand. Drop-on-demand systems are generally lower cost but relatively low print speed when compared to continuous systems. In either drop-on-demand or continuous inkjet systems, it is necessary to assign a different fluid ink color to a separate printhead. Therefore, in color prints, several layers of wet ink may be deposited onto a printed medium.
Traditional printing presses are able to use high viscosity inks to obtain vibrant, high-density colors. However, continuous ink jet systems employ low viscosity solutions of dyes or pigments in a water solvent, and the printed colors tend to not be as vibrant and dense as with other printing systems. It is known that increasing the amount of dye or pigment applied to the paper can brighten the colors. However, this process also increases the amount water solvent applied to, and absorbed by, the paper. Absorption of water may cause a paper wrinkling effect called cockle, a wicking and spread of colors referred to as color-to-color bleed, and/or a show-through to the back side of the paper.
To remove water from the printed medium, continuous systems have conventionally utilized an end-of-line dryer that is similar to those used in printing presses. See for example U.S. Pat. No. 5,423,260 issued to Rockwell International Corporation in 1995, wherein the end-of-line dryer removes water from the printed medium only when all wet ink has been deposited and is at its maximum. It has been suggested to use infrared lamps or microwave radiation to preferentially heat the ink relative to the unprinted receiver media. However, tests have shown that dryers consisting of infrared lamps or microwave radiation cause a significant amount of receiver media heating to occur.
Further reductions in the time required between printing and drying have been realized by placing dryers between two printheads to dry the ink before significant amounts of the ink can wick into or otherwise be absorbed by the receiver media. Placement of dryers between printheads is referred to herein as “inter-station drying,” and has been disclosed in U.S. Pat. No. 6,428,160B2, issued to Xerox in 2002. Inter-station drying is effective to provide better optical density, sharper edges, less show through and reduced cockle. In multi-color systems, high-speed dryers placed between the different color printheads reduce color-to-color bleed, and enable more ink to be employed without overly wetting the receiver media. U.S. Pat. No. 5,631,685 discusses these benefits in relationship to single color printers. JP07-314661 speaks of these benefits for a multi-color inkjet printer. U.S. Pat. No. 6,428,160B2 addresses the paper scorching issues by selectively heating only the ink and not the paper. However, selective heating of the ink may create a saturated boundary layer at the ink surface. That is, as heat is directed to the newly applied ink, water evaporates rapidly from the surface of the ink, forming a thin layer of saturated air just above the ink. Therefore, it has been found necessary to include a mechanism for removing the saturated air layer just above the ink spot.
It has been suggested to remove the saturated air layer using a combination of convection and radiation. U.S. Pat. No. 5,261,166 discloses a dryer comprising a plurality of infrared burner units with air floatation dryer elements between the infrared units. The air floatation elements mentioned in the patent are of the Coanda type. U.S. Pat. No. 6,412,190 also employs infrared burners in conjunction with air bars. U.S. Pat. No. 6,088,930 employs alternating infrared sources and blower units. Suction nozzles are located between the infrared sources and the blower units to remove air from the blower regions. This patent discloses the concept of reflectors being placed on the opposite side of the paper from the infrared sources to reflect the radiation back at the paper. WO 88/07103 describes a dryer unit in which the lamp used for generation of infrared radiations enclosed in a box with a reflector behind the lamp and an infrared transmitting window in front of the lamp. Air is directed through the box to cool the lamp, the reflector, and the inner surface of the window. This air exits the box by way of a Coanda slot that causes the air to be directed between the window and the paper. U.S. Pat. No. 5,092,059 describes a dryer unit in which an infrared source directs infrared at the paper through a Quartz window. Coanda slots located on two sides of infrared source cause air to flow between the window and the paper to remove moist air from this space. Commonly assigned U.S. Pat. No. 6,058,621 describes a dryer in which a plurality of radiant heating bars direct radiation at photosensitive paper. Reflectors are placed behind the infrared lamps. Air flows out between the reflectors, impinging on the paper.
Air bearing systems allow for contact-less support of a print media, especially web-like materials. This contact-less support is sometimes crucial to ensure that the web or print is not damaged. The air bearing condition is traditionally created by deflecting the trajectories of the air molecules immediately adjacent to the print media in a direction parallel to the movement of the printed medium. The parallel movement of the air molecules thus establishes a cushion of air providing support for the printed medium. For example, U.S. Pat. No. 3,324,570 issued in 1967 teaches a float dryer developed for fabrics. A more recent adaptation of the 1967 patent, U.S. Pat. No. 5,261,166 issued to WR Grace in 1993, used a combination infrared and air flotation dryer. WR Grace uses a combination of their HI-FLOAT® air bar in combination with an infrared gas burner, INFRAWAVE® by Maxon Corporation, to create a fast dryer that removes the saturated boundary layer by impinging air upon the ink surface. The end-line dryer taught by WR Grace requires that all fluid inks be placed onto the printed media web prior to initiation of drying.
The patents described above utilize infrared radiation to provide the energy transfer needed for effective drying combined with air bearing features to enhance the transfer of moist air away from the paper. None of the prior art used a microporous filter air bearing design, as is the case of the present invention, but rather used either Coanda type or air bar types of air bearings. While Coanda type or air bar types of air bearings are effective to handle large air volumes and velocities, the air flow is directed toward a common point, which causes a wet image to smear at the air impingement point. It would be advantageous to allow for a diffuse and more controlled overall air flow without loosing the capacity for large air flow or volume.
It is an object of the present invention to provide an inter-station drying system such that the benefit of rapid drying of printed ink or other water based liquid without the creation of a saturated boundary layer issue by supplying a large volume and high velocity air flow such that air flow prevents overheating without creating additional smear, and to rapidly cool the substrate by removing any residual heat generated by the radiation source.
It is another object of the present invention to provide a dryer system to be used in close proximity and in series with at least one inkjet printhead or water based liquid applicator to include a source of heat, a source of air flow, and a structure in communication with the air flow that converts the air flow to a substantially diffuse flow compatible with printed, wet inks. The diffuse flow of air is such as to create a cushion of air at the surface of the receiver medium.
It is still another object of the present invention to arrange air sources along the printed medium and on both sides of the receiver medium in a manner to provide a contact-less receiver medium support.
It is yet another object of the present invention to layer the heat source and the gas source to minimize the overall length of the printing system.